In the field of ferrochromium production, pre-reduction of chromite ores prior to final smelting in electric furnaces is becoming of increasing importance in order to decrease the use of costly electrical energy.
Rotary kilns or rotary hearths are typically used for this pre-reduction process, with the heat requirements being supplied by combustion of coal, gas or oil and the reduction of the ore by coal, char or coke. In the typical existing processes, finely ground chromite ore is pelletised together with coke and fed with an excess of coal to a rotary kiln operating at temperatures of up to 1450.degree. C. These processes do not achieve complete reduction of the chromite and the partly-reduced pellets are then charged to an electric submerged arc furnace for final smelting reduction and separation of molten ferrochromium from the slag.
In a more recent process, described in West German Patent DE No. 3347/686 CI to Fried. Krupp GmbH, chromite ore fines are fed directly to a rotary kiln together with excess coal and appropriate fluxes, and treated at temperatures up to and exceeding 1500 C. A nearly completely reduced product results (more than 90% metallisation of the chromium and nearly 100% metallisation of the iron present). This product is suitable for charging directly to a melting unit, such as, for example, an electric arc furnace, to separate the ferrochromium from the slag, thereby avoiding the usual reduction smelting operation.
All of these pre-reduction processes tend to result in high sulphur contents in the metallic fraction (typically 0,25% S) due to the use of large amounts of coal or coke, which partly replace the electrical energy requirements in addition to providing the reductant for the necessary chemical reactions. These carbonaceous materials are the main source of sulphur in ferrochromium production. Sulphur is a deleterious impurity in steels and as ferrochromium is a basic ingredient of chromium-containing steels, such as stainless steels, strict limits are placed upon the maximum contents of sulphur in ferrochromium. Typical upper limits range from 0,03% S to 0,05% S.
According to the Krupp patent already referred to, desulphurisation can be carried out by injecting lime or calcium carbide into the pre-reduced ferrochromium after it has been melted in a suitable furnace. It is claimed that by this means the sulphur content of the ferrochromium can be lowered to less than 0,01%. Although it is well known that both lime and calcium carbide are effective desulphurising agents in the iron and steel industry, desulphurisation of ferrochromium by powdered reagent injection into the melt has not yet proved to be satisfactory for both technical and economic reasons.
The object of this invention is to provide a more simple and more cost effective means of desulphurising the ferrochromium from the aforesaid pre-reduction process, and also to provide some control over the final silicon content of the ferrochromium.